In the semiconductor industry, smaller and faster devices are in constant demand. In complementary metal oxide silicon (CMOS) technology, a need to enhance the speed and increase the density of CMOS integrated circuits (ICs) has resulted in the evolution of transistor scaling, accompanied by a progressively thinner gate dielectric, typically an oxide. A reduction in the thickness of the gate dielectric provides increased drive current, thereby resulting in an increase in transistor speed. In addition, a thinner gate dielectric offers enhanced control of channel charge, thereby reducing short channel effects. The fabrication of thinner gate oxides, however, presents gate leakage current problems and reliability issues. In particular, physically thinner gate oxides exhibit a gate leakage current which increases exponentially with the reduction in thickness.
The gate leakage current can be mitigated by introducing nitrogen atoms into the gate dielectric. Introducing nitrogen to the gate dielectric generally reduces the gate leakage current through the gate dielectric increasing the dielectric constant of the gate dielectric. One method of nitrogen atom introduction is to perform non-thermal nitridation (e.g., plasma nitridation) on the gate dielectric. Exposure of the substrate to air and airborne molecular contaminants (AMCs), such as organic hydrocarbons, prior to the nitridation of the gate dielectric, however, can result in inadvertent non-uniformities in the nitridation of the gate dielectric, therein causing non-uniformities in gate leakage current associated therewith. Such inadvertent non-uniformities can deleteriously increase an equivalent oxide thickness (EOT) of the gate dielectric.
Accordingly, AMCs can cause an increased and variable dielectric constant (and hence, EOT variability) across a surface of a single wafer depending on the location of the AMCs across the surface, as well as causing EOT variabilities from wafer to wafer. Variablilites in dielectric constants caused by AMCs are especially prevalent as EOTs are reduced below about 20 Å. Furthermore, AMCs typically cause an increased surface roughness of the gate oxide, wherein the roughness in the gate oxide film can further degrade the performance of the subsequently nitrided gate dielectric. Therefore, a need exists in the art for a process for generally eliminating AMCs prior to processes such as plasma nitridation.